A concrete member is not resistant to a tensile force, but is resistant to a compression force. In consideration of this characteristic of the concrete member, so-called prestressing for applying a compression force to a concrete structure beforehand is known (for example, see Patent Document 1).
FIG. 5 is a process explanatory view illustrating a typical prestressing method. Among prestressing methods, the method shown in the figure is called post tensioning. To form a prestressed concrete structure (PC structure) by prestressing, first, a cast form 20 is placed (see FIG. 5(A)), anchor plates P are placed on opposite sides of the cast form 20, and a buried pipe (bore hole) BP is placed in the cast form 20 (see FIG. 5(B)). Next, concrete is cast in the cast form 20. After the concrete sets, a strand 100 is inserted into the buried pipe BP (see FIG. 5(C)). Next, the strand 100 is tensioned at both ends of a concrete block CB, and the tensioned strand 100 is fixed at both end faces of the concrete block CB by anchors AD each including a wedge, an anchor head, and an anchor plate (see FIG. 5(D)). Then, the buried pipe BP is filled with grout, whereby a PC structure is completed (see FIG. 5(E)).
Here, to fill the above-described buried pipe BP with grout, a grout injection hole and an air discharging hole (grout discharging hole) that extend from the anchor plates to the buried pipe BP are used (see FIGS. 5(C) and 5(D)). Specifically, grout is injected into the buried pipe BP through a conduit 110, and the grout is discharged from the buried pipe BP through a conduit 120, thereby filling the buried pipe BP with the grout (see directions shown by the arrows in FIG. 5(D)).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-350736